Mischief: by Charlotte Armstrong.

Mischief begins with a stranger in the house, with children in the house (I am uncertain of the details, after all these years it is a mist veiled memory), the stranger is supposed to baby sit, and seems very quiet, demure, but the mother of the children is uneasy, for no discernible reason that she can think of; so the couple go out for the evening.

And then, slowly, the young woman now no longer demure or quiet, runs amok, just for fun, just mischief - so much so the guy who meets her for fun is afraid. There is an eight year old child, a girl, in danger.

Fortunately of course the mother's instinct wins over the rational edicts of the day to put the couple's sex life first and she rushes home, saving her child in the process.

It is obviously about cautioning people about letting in strangers in their homes, and what havoc can then be let loose even by young people out to make mischief just for fun.